Posted on Jul 13, 2012 10124
This is a sensitive, mini-powered FM transmitter consisting of an rf oscillator section interfaced with a high-sensitivity wide passband audio amplifier and capacitance microphone with built-in FET that modulates the base of the rf oscillator transistor. The setting of C8 determines the desired operating frequency- in the standard FM broadcast band, tuned to favor the high end up to 110 MHz. Capacitor C7 supplies the necessary feedback voltage developed across Rll in the emitter circuit of Q2, sustaining an oscillating condition.
Resistors R9 and RIO provide the necessary bias of the base-emitter junction for proper operation, and capacitorClO bypasses any rf to ground fed through to the base circuit. C9 provides an rf return path for the tank circuit of Ll and C8, while blocking the de supply voltage fed to the collector of Q2. The speech voltage developed across Rl by Ml is capacitively coupled by C4 to the base of Ql. A signal voltage developed across R4 is capacity-coupled through C6 to the base of Q2 through R8. R7;md R8, along with Cl and C2, decouple the oscillator and audio circuits.